The present invention relates generally to an image output device referred to as a raster device which represents images provided by a printer, facsimile, CRT and like, as a set of dots or pixels, and more particularly to digital halftoning techniques for realizing image representations in a higher resolution and with a larger number of halftone levels of gradation.
Conventionally, raster devices having a small number of halftone levels expressible by one pixel, such as a digital printer, have utilized for many cases halftone representation measures known as a dither method, an error diffusion method, or the like to simulatively compensate for halftones.
Within these conventional methods, the dither method is widely utilized since it is easily implemented and provides a relatively uniform image quality although it is inferior to the error diffusion method in terms of the gradation and resolution.
The dither method sequentially compares each point in an input image matrix with each element in a threshold matrix referred to as a dither matrix to determine an actual output halftone level of a corresponding pixel. Particularly, a device, which provides an output image at two levels of gradation, determines whether a pixel corresponding to a point is on or off.
As an example of such a dither matrix, an article "An Optimum Method For Two-level Rendition Of Continuous-halftone Pictures" by B. E. Bayer (ICC Conf. Rec. 26-11.about.15), 1973 discloses patterns shown in FIGS. 1A and 1B. The dither pattern shown in FIG. 1A is a distribution type dither pattern commonly known as a Bayer-type pattern. The pattern shown in FIG. 1B is employed by a dot concentration type dither method which utilizes a plurality of dots to simulate fat dots, the diameter of which is increased corresponding to a halftone level.
The application of the dither pattern shown in FIG. 1A will result in an output image, though its resolution is high, having a pattern giving a peculiar coarse feeling which is highly prominent. On the other hand, in an output image generated by the application of the dither pattern shown in FIG. 1B, an eyesore characteristic pattern will not be produced in the output image since fat dots having diameters corresponding to halftones are orderly arranged at regular intervals. However, since the resolution of the output image is determined by the intervals of the fat dots, the interval of the fat dots becomes wider if a larger number of halftone levels are provided for images, and a sufficient number of halftone levels cannot be ensured for images if the fat dots are to be arranged at narrower intervals.
To cope with the problems mentioned above, there has conventionally been proposed a method for arranging a plurality of fat dots in a sequence of threshold values in a dither matrix. For generating such a dither matrix, methods for applying a Bayer pattern generation algorithm shown in FIG. 1A to a dot concentration type dither have been often utilized, as described in a report entitled "Threshold Arranging Method in Dot Concentration Type Dither Method" by Ueno et al. in 1980 General National Meeting of Japanese Institute of Electronics, Information and Communication Engineers.
Specifically, the above-mentioned Bayer's extension algorithm generates n patterns of the same form nD+0, nD+1, . . . , nD+(n-1) (generally, n=2 or 4) from a basic dot concentration type dither pattern D, and combines the thus generated patterns to extend the dither pattern.
Actually, dither patterns disclosed as FIGS. 16 and 17 in JP-A-61-125264, and many dither patterns shown in Japanese article by Kawamura et al. "Halftone Reproducing Method for Digital Color Printing in Electronic Photography (III)", in Transactions of The Institute of Image Electronics Engineers of Japan, Vol 25, No. 1 (1986) are generated by methods as mentioned above. In addition, JP-A-58-173973 discloses a rather irregular method.
A fat dot density of the dot concentration type dither pattern is measured in lines per inch (lpi). When a human observes an output image at a distance of approximately 30 cm, the fat dot density in a range from 100 lpi to 120 lpi exceeds the resolution of human so that an image having this range of fat dot density is recognized as a smooth image. Stated another way, a sudden qualitative change occurs in an output image with the fat dot density ranging between 100 lpi and 120 lpi.
Correspondingly, since discontinuous halftones are more likely to be prominent in a smooth image corresponding to the fat dot density of approximately 120 lpi, the application of a dither pattern having a less number of halftone levels will result in a rather unnatural image. A number of halftone levels required in this case is also considered based on approximately 120.
Therefore, if a density of fat dots equal to 120 lpi is to be ensured in a binary raster device having a resolution of 600 dpi (dot per inch) such as a high definition laser beam printer or the like, which is commonly available at present, a fat dot is composed of 5 by 5 pixels (25 halftone levels) or less, so that a dither pattern must be composed of five or more fat dots.
Conversely, however, as the number of fat dots simulated in a dither pattern is increased, an irregular and coarse dot arrangement may be generated in a lower halftone level representation, that is, an area of highlight, where the centers of all dots are not aligned, thus, damaging the quality of a resulting output image, as described in JP-A-58-173973.
As is understood from the above, the smoothness of images is more or less sacrificed in the prior art. Specifically, the number of fat dots in a dither pattern is limited to two-four, as disclosed in the Kawamura's article and JP-A-61-1255264. Alternatively, a plurality of dither matrices are prepared such that a different matrix is used for inputted pixels in a lower halftone area, as disclosed in JP-A-58-173973.
The method described in JP-A-58-173973, however, has a problem that complicated memories, associated circuits and programs must be configured for preparing a plurality of dither matrices, thus causing a lower processing speed and an increased cost. Particularly, it has been found that the continuity of halftone or gradation is degraded.
The method described in JP-A-61-125264 also has a problem that the number of fat dots set to four may cause a prominent degradation in image quality due to irregular dots in a lower halftone area, and that the number of fat dots set to two may result in a largely damaged gradation. Conversely, if a number of halftone levels equal to or more than 120 is to be ensured using two fat dots, an 8.times.8 image is required for one fat dot, so that a coarse image having a density of fat dots equal to 75 lpi will be generated.
In view of an improvement in the resolution of an output device, assuming that a number of halftone levels equal to or more than 120 is to be realized with four fat dots, as disclosed in JP-A-61-1255264, even if a fat dot is composed of 6.times.6 pixels, the output device is required to have a resolution of 720 dpi (120.times.6=120) in order to produce images having a screen line number of 120 lpi. With a device, for example, a page printer, which temporarily stores a complete output image in a buffer, an increase in memory buffer capacity of as much as 20% will give rise to a seriously increased cost.